The present invention concerns a rod-bed assembly and, to be more precise, a rod-bed assembly for a device for coating a material web, particularly a paper or cardboard web.
When coating paper or cardboard in a paper machine, it is necessary to apply a liquid coating medium, such as a coating material, onto the paper or cardboard web running through the paper machine. This is usually effected by an excessive application of the coating medium and by a subsequent reduction or dosing of the coating medium to the desired quantity. The dosing of the coating medium is usually carried out by a dosing apparatus, such as a rod-bed assembly having a doctor rod as dosing element for scraping off the excessive material quantity.
Such a rod-bed assembly is known from EP 1 485 209 A1 or WO 03/078077 A1 where a metal doctor rod is insertable into a rod bed made of an elastomer. The rod bed is inserted into a holding groove of a holder also made of an elastomer. In order to ensure a safe hold of the doctor rod, the holder is formed as a kind of bracket in combination with a pressure hose which continuously presses against a bracket leg during the operation of the rod-bed assembly for fixing the doctor rod. This serves to press the groove walls of the holder groove against each other in a direction towards the rod bed, so that the inserted doctor rod is rotatably held over the rod bed. Additionally, a further pressure hose is provided on the rear side, i.e. on a side of the holder facing away from the holder groove, enabling a flexible movement of the holder so as to adjust a position of the doctor rod with respect to the paper web to be coated.
It is a problem of this state of the art that the pressure hose pressing against the one bracket leg has to be under constant pressure during the operation of the rod-bed assembly. Moreover, a further pressure hose has to be provided for effecting the adjustment of the doctor rod.
A further rod-bed assembly is known from WO 2007/063183 A1, which basically shows a similar construction as the above-described rod-bed assembly. In this case, however, no pressure hose for securing the doctor rod is used. Instead, the rod bed has been given a special outer shape with a tapering cross-section, by which a securing of the doctor rod in the flexible rod bed is achieved by pressing the doctor rod into the rod bed inserted in the holder. This means that the rod bed has a self-securing function due to the pressed-in doctor rod in the holder; said function can be further reinforced in operation by the doctor rod being pressed on the paper web.
It is a problem of the aforementioned prior art that the rod bed of the rod-bed assembly is a wearing part that has to be replaced regularly. This, however, is made difficult by the doctor rod being pressed into the rod bed for being secured in the holder, thereby “wedging” the rod bed in the holder. It is very complicated and time-consuming to remove the doctor rod from the rod bed, and the rod bed from the holding groove. Additionally, the rod bed is carved out of solid matter, wherein machining traces may occur in the machined surfaces, among others in the sliding bearing of the doctor rod, which negatively affect the sliding properties of the doctor rod. Besides, a lubricant for lubricating the doctor rod can pass through the machining traces and mix with the coating medium, thereby impairing the quality of the same. It is also conceivable that the coating material settles in the machining traces and dries there, which may disturb a continuous coating medium film on the doctor rod, or even increase a wear of the doctor rod.
As a further example for a known rod-bed assembly, DE 100 45 515 A1 shows a doctor rod-dosing-assembly for paper or cardboard web coating devices, which comprises a holder, a doctor bed inserted into the holder and made according to a continuous primary molding method, and a doctor rod rotatably supported in the doctor bed.